/Engine/Source/Editor/EditorFramework/Public/Toolkits/AssetEditorModeUILayer.h
Called by SLevelEditor to notify the toolbox about an existing toolkit no longer being hosted
virtual void OnToolkitHostingFinished ( const [TSharedRef](API\Runtime\Core\Templates\TSharedRef)< [IToolkit](API\Editor\EditorFramework\Toolkits\IToolkit) > & Toolkit )